Abstract
It is widely recognized that nitrogen (N) inputs from watersheds to estuaries are modified during transport through river networks, but changes within tidal freshwater zones (TFZs) have been largely overlooked. This paper sheds new light on the role that TFZs play in modifying the timing and forms of N inputs to estuaries by (1) characterizing spatial and temporal variability of N concentrations and forms in the TFZs of the Mission and Aransas rivers, Texas, USA, and (2) examining seasonal fluxes of N into and out of the Aransas River TFZ. Median concentrations of dissolved inorganic N (DIN) were lower in the TFZs than in upstream non-tidal river reaches and exhibited spatial gradients linked to locations of major N inputs. These spatial patterns were stronger during winter than summer. The forms of N also changed substantially, with DIN changing to organic N (primarily phytoplankton) within the TFZs. Discharge and N flux comparisons for the Aransas River TFZ demonstrated that secular tidal patterns modulate the timing of N export during baseflow conditions: N export far exceeded input during winter, whereas export and input were relatively balanced during summer. While more data are needed to build an annual N budget, our results show that TFZ can change the timing and form of N export immediately upstream of estuaries.
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Acknowledgements
We thank the REUisME program (NSF #1358890) led by Dr. Deana Erdner and the University of Texas at Austin Semester-by-the-Sea program. Both programs supported undergraduate researchers Grayson Barker, Kylie Holt, Spyder Julian, Tricia Light, Sierra Melton, and Ana Salamanca, who assisted with this project in the field and lab. We also thank Dr. Ryan Hladyniuk and Patricia Garlough for analyzing the particulate organic nutrient samples. This work would not have been possible without their help.
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This study is supported by the National Science Foundation (#1417433).
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Amber Hardison, James McClelland, and Kevan Moffett conceived and designed the research, Hengchen Wei and Xin Xu collected the biogeochemical data, Allan Jones measured and modeled the discharge data, all authors contributed to the data analysis, Hengchen Wei wrote the original draft, and all authors contributed to the revision of the manuscript.
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Wei, H., Xu, X., Jones, A.E. et al. Tidal Freshwater Zones Modify the Forms and Timing of Nitrogen Export from Rivers to Estuaries. Estuaries and Coasts 45, 2414–2427 (2022). https://doi.org/10.1007/s12237-022-01112-7
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DOI: https://doi.org/10.1007/s12237-022-01112-7